US-based Neil Matheson became CEO of Huntsworth Health in 2009, having previously led Axis Healthcare Communications and before that ApotheCom.
He has grown Huntsworth Health from a unit contributing a quarter of the wider group's revenue, to one which accounted for just over half of it in 2016 - when it was also the fastest growing business of the group, with like-for-like revenue up 13.8 per cent to £90.8m.
Yesterday, Huntsworth announced that its health division had again been its strongest performer in the first half of the year, achieving like-for-like growth of nearly 20 per cent, firmly on track to exceed £100m revenue for the full year.
Huntsworth CEO Paul Taaffe said: "Neil has left the business with our enormous thanks. He set out what he wanted to achieve when I was appointed CEO just over two years ago and, as our half year results showed, mission accomplished.
"All his colleagues at Huntsworth are grateful and wish him well on his next venture."
It is not clear whether a direct successor will be appointed.
At the start of the month, Huntsworth acquired The Creative Engagement Group, a three-brand UK group predominantly focused on healthcare clients.
Meanwhile, Tonic Life has recently experienced leadership changes, with Maryellen Royle promoted to global CEO in October following the departure of Mary Smiddy, and a new MD hired in its Philadelphia office at the start of the year. Also in October, Jon Clark was appointed CEO of Nitrogen.
Other Huntsworth Health brands are ApotheCom, Audacity, Axiom and Evoke Health.